Super innocent example: A 14 year old kid breaks their leg and the cops show up to them in agony and crying. Would be very easy for a few people that dislike them to plaster them crying all over social media. Yes, the event was public to begin with, but the chances of one of the bullies walking by and snapping their own photos is almost nil.
A lot of people that are interacting with the police are in a very vulnerable state, it would be asinine to expose that to the world. The thought that it will create "good will" with the public is entirely backwards in my opinion, if I'm in trouble and I know if I get the cops that I may end up all over the internet it may keep me from calling them at all.